Big, expensive houses, the main-stay of the real estate market in Potomac, McLean, Great Falls and other fashionable Washington neighborhoods, are turning into this summer's real estate headache.

An unprecedented glut of houses for sale for $150,000 and up has developed in the metropolitan area, slowing down the pace of sales and forcing owners to postpone moving.

In Montgomery Country alone, according to data prepared by real estate consultant Alfred W. Jarchow, there are twice as many houses for $100,000 and up on the market as there were a year ago. The 1,000 expensive houses currently for sale in the country represent nine months worth of potential transactions, three times the number considered healthy for the residential real estate business.

Homes at the lower end of the price spectrum, however, have been selling rapadily all year, and a shortage exists, Jarchow says. In Montgomery, units selling for $50,000 and under -- primarily town houses and condominium apartments -- are down to a fourth of the number availalble a year ago. In the $50,000 to $80,000 range, there are enough units for about two months' worth of sales, compared with the three month's supply relators like to have available.

The same phenomenon is felt across the river in Northern Virginia's best-heeled suburbs, and to a lesser extent, in the District. Brokers report that they're loaded down with high-priced -- and sometimes overpriced -- houses but can't find anthing to list or sell that's moderately priced.

"i've never seen anthing quite like this before in the higher-price categories," said Caroline Campbell, a veteran broker in Great Falls. "There's just too much on the market at a time when demand is heading downwards temporarily." Cambell is recognized by the Northern Virginia Board of Realtors as a specialist in $150,000-to-$500,000 residential properities in the McLean-Great Falls area.

Sales were "extraordinarily good during the winter and early spring," she noted, "but a backlog began developing a couple of months ago, and now it is very large indeed.

"We can sell all those houses," she added "but it's going to take more time and effort than sellers expected."

In the District, the market for houses costing $175,000 and up has also softened considerably in the past tow months, brokers report. In the early spring, the city was one of this area's high-cost hot spots, with one out of 10 houses going for $200,000 or more.

Now, it's more restrained market, and the wild upward spiral of prices, which rose by an average of 30 percent in the upper price bracket last year, appears to be flattening out. The decision by the City Council this week to suspend the 11 percent unsury rate on first trusts and mortgages for 90 days may pep up the market again, but will not restore it to its earlier pace.

The causes of the "great glut of 1979," as one Montgomery broker calls it, are a matter of speculation. But some of the factors unquestionably at work include consumer resistance to the inflated asking prices of many sellers in the upper brackets, as well as resistance to high interest rates and the huge monthly payments these require. An 11 percent, $1500,000 mortgage for 25 years requires a monthly payment of $1,471, esclusive of property taxes.

Demand for high-price houses also tends to be more sensitive to national economic swings. On the verge of a recession, with 14 percent inflation and real income levels on the decline, many buyers have become catious. Energy-inefficient houses in the farout suburbs may look less appealing at a time like this.

An unrecognized but important factor in the big-home market this year, however, may be the $100,000, tax-free capital gains exclusion that Congress wrote into the Internal Revenue Code last year. It allows homeowners who are 55 years or older to avoid taxation on up to $100,000 of the profit on the sale of their principal residence.

The law was expected to free up some of the larger houses that older families couldn't have afforded to leave because the capital gains burden. But local brokers such as Caroline Campbell report that a surprising number of the big, high-priced houses sitting on the market now are there solely because of the $100,000 tax-free opportunity. There are so many of them that sellers are finding it hard to get full asking prices.

Among her listings in the $200,000 bracket, Campbell said, several are there "for no other reason than the owners' desire to cash out now, tax-free, before Congress changes its mind." The sellers intended to take their profits and buy -- you guessed it -- moderate priced condominiums.

Kenneth R. Harney is executive editir Reporter, published by BNA, CAPTION: Picture, House like this under construction in McLean, part of a development where prices range from $166,000 to $183,000, are proving harder to sell now. By Jack Hayes for The Washington Post